It is well recognized that the solar ultraviolet (UV) radiation poses serious threat of human skin damage which may range from the short term hazard like erythema, i.e., sunburn to long term hazards like skin cancer and/or premature aging of the skin. The UV radiation having a wavelength of from 290 nm to 320 nm, generally referred to as the UVB radiation, is what is known to cause erythema. On the other hand, numerous reported studies point to unprotected skin exposure to the UV radiation having a wavelength of from 320 nm to 400 nm as being the primary cause of skin cancer. It is essential, therefore, that the skin is protected from both UVA and UVB radiations in order to avoid long and short term deleterious effects of solar radiation.
The SPF (Sun Protection Factor) rating system has been developed to help consumers select the appropriate sun protection product for any given outdoor activity involving exposure to the sun. The SPF number corresponds to the multiplying factor by which the duration of protection by a properly applied sunscreen exceeds the exposure time that causes the unprotected skin to show darkening. Thus, with proper application of an SPF 15 product, a person should be able to remain in the sun without skin darkening for fifteen times the usual unprotected duration.
In recent years, due to the increased public awareness of the aforementioned hazards of UV radiation, the use of sun protection products has grown considerably, with consumers preferring products that offer protection over the entire range of UV radiation, i.e., from 290 nm to 400 nm. Nonetheless, while there are several UVB absorbing materials approved for use in sun protection products by the regulatory agencies in different countries, the number of approved UVA absorbers is far less. Among these approved UVA sunscreens is the amphoteric metal oxide, zinc oxide (ZnO).
Most personal care and cosmetic products in the form of lotions and creams are essentially either oil-in-water (O-W) or water-in-oil (W-O) emulsions, with the majority being O-W emulsions. Most of these O-W emulsion-based compositions contain various additives including emulsifiers and thickening agents, many of which bear negatively-charged, i.e., anionic functional groups. Adding positively-charged, i.e., cationic ingredients such as cationic surfactants, polymers, and particulate solids at a relatively high dosage to emulsions that contain anionic emulsifiers and thickeners poses a high risk of destabilizing the emulsions. The electrostatic attraction between oppositely charged moieties mixed in a formulation may render these ingredients functionally ineffective and/or insoluble, which in turn is bound to produce unstable emulsions.
Such instability generally manifests in various forms including separation of the oil phase and/or the water phase of the emulsion, precipitation of otherwise soluble materials, separation of solid particulate materials otherwise meant to remain suspended in the emulsion, undesirable thinning of the emulsion, excessive thickening of the emulsion, and pH-shift. Some of these instability problems may arise also if the water phase of an O-W emulsion contains relatively high amounts of dissolved cations, especially the multivalent cations, along with substantial quantities of anionic and/or electrolyte-sensitive additives.
Zinc oxide, available in the form of a finely milled particulate material, exhibits properties that present considerable challenge in producing stable O-W emulsions that were to contain anionic and/or electrolyte-sensitive additives. Over a wide range of pH, ZnO particles tend to have a cationic surface charge, when suspended in water or an electrolyte solution. Also, the pH of minimum solubility for zinc cations is considerably high, in the range of 9.5-10.5, implying that increased dissolution of zinc cations from zinc oxide solids may occur between the pH of 6 and 9, the typical pH range for most personal care products. Adding further complexity, solubilization of zinc cations from zinc oxide may be enhanced in the presence of certain anionic surfactants having weak acid groups such as phosphate, carboxylate, phenolate, and phosphonate groups, which are often used as anionic emulsifiers for O-W emulsions.
Unless countermanded, the aforementioned detrimental properties of ZnO would cast a stronger influence if the ZnO particles are dispersed in the water phase of O-W emulsions. In other words, the adverse effects of ZnO on emulsion stability could be minimized if the ZnO particles are contained in the oil phase of the emulsion and thus precluded from entering the water phase of the emulsion, as taught in the U.S. Pat. No. 6,464,965 B1. However, since the surface of these particles is hydrophilic, dispersing them in the oil phase would require hydrophobic modification of the surface via covalent-bonding of hydrophobic materials (e.g., silane), and/or by using wetting agents or dispersing agents. These approaches, however, would add considerable cost, yet might fail to provide sound reliability, and could lead to various other formulation problems, for reasons such as the following:    i) During bulk manufacturing using ordinary solid-liquid mixing processes, it would be impossible to ensure complete surface-coverage of all primary (single) ZnO particles contained in a given weight of ZnO solids, via covalent-bonding of hydrophobic surface modifying agents.    ii) Such coated ZnO would be considerably more expensive than pristine ZnO.    iii) The more-effective wetting or dispersing agents that might be used for dispersing pristine ZnO particles in hydrophobic or oily liquids would be the surfactants and/or the polymers that also find use as emulsifiers for water-in-oil (W-O) emulsions. The presence of excess amounts (corresponding to relatively high dosage levels of ZnO) of these W-O emulsifiers in O-W sunscreen emulsions may lead to difficulties in stabilizing the emulsions, especially if the emulsions' oil phase content is relatively high, as might be in high-SPF sunscreen products. Increasing the dosage of the O-W emulsifiers might help in counteracting the effects of the W-O emulsifiers, but the concomitant increase in detergency due to the O-W emulsifiers is likely to present problems in achieving good water-repellency for the sunscreen, a highly desirable feature for most sun protection products.
Yet another way of avoiding the instability problems induced by ZnO would be to coat the ZnO particles with other hydrophilic, inorganic oxide materials, e.g., titanium dioxide and silicon dioxide, which, unlike ZnO, are not likely to hamper the stability of O-W emulsions. These coated ZnO materials, however, would be considerably more expensive than pristine ZnO.
The U.S. Pat. No. 6,500,411 B2 discloses the use of particulate materials including ZnO for boosting the SPF of emulsion-based sunscreen compositions, requiring that the particulate materials remain dispersed in the water phase of the emulsions, and that certain phenolic polymers render the particles capable of resisting agglomeration or flocculation in these compositions. The invention demonstrates that aqueous dispersions comprised of particulate materials such as titanium dioxide (TiO2), particulate-based thickeners such as smectite clays, and certain water-soluble phenolic polymers such as lignosulfonate used as a dispersing agent for the suspended solids, when added to sunscreen emulsions containing at least one organic UV absorber, increase the SPF largely. In contrast, similar aqueous dispersions of the aforementioned particulate solids, but without any lignosulfonate contained therein, produce no SPF boost. Also, substituting sodium polyacrylate for lignosulfonate as the dispersing agent greatly reduces the SPF boost. The invention, however, does not specify nor address any issues concerning the stability of the sunscreen compositions disclosed therein.
Nonetheless, it was found during the course of the research leading to the present invention that ZnO-containing sunscreen compositions of the type described in the aforementioned patent exhibited an extraordinary behavior in terms of stability inasmuch as their stability seemed to depend on the manufacturing process, although in a counterintuitive sense. An O-W sunscreen emulsion that contained in its water phase 2% by weight of ZnO exhibited poor stability, when produced using a high-shear homogenizer. On the other hand, the same emulsion could be manufactured in a more stable form when an agitator was used instead during emulsion-making. Typically, a high-shear homogenizer is more effective than an agitator in producing stable emulsions. Also, the instability manifested as the separation of the water phase as localized or spotty water pools, appearing mostly at the surface of the emulsion, rather than of the oil phase of the emulsions, whereas an unstable O-W emulsion would typically show the reverse.
It is speculated that this peculiar stability behavior of the foregoing emulsion is due to phenomena such as the following: Air is entrapped within the emulsion during its manufacturing, which takes on the form of ultra-fine air bubbles in the presence of a moderately foaming polymer such as lignosulfonate and under the action of a high-shear homogenizer. Under the destabilizing influence of ZnO, these ultra-fine air bubbles undergo coalescence ultimately, thus growing bigger in size. Eventually, the collapsing of numerous air bubbles upon reaching the surface of the emulsion sample produces the spotty liquid pools at the emulsion surface.
Even though the technology disclosed in the U.S. Pat. No. 6,500,411 B2 is currently being used widely for producing commercial sunscreen products that do not contain ZnO, its use in producing ZnO-laden sunscreen products may have limited commercial viability because of the instability issues described above.
An object of the present invention, therefore, is to provide ZnO-containing O-W sunscreen emulsion compositions which exhibit a high level of physicochemical stability, yet not requiring i) the ZnO particles to be coated with either any other water-insoluble inorganic material or water-insoluble organic material; and ii) the ZnO particles to be contained substantially in the oil phase of the emulsions. A related object is to obtain ZnO-laden, yet, highly stable O-W sunscreen emulsion compositions wherein the ZnO particles remain dispersed in the water phase of the emulsions, preferably in a highly deflocculated form.
Most commercial sunscreen products utilize high levels of organic UV absorbers which tend to be expensive and are oil-like and/or oil-soluble materials, in achieving high SPF values. These high levels of sunscreen actives increase the cost of the product, while rendering the product less appealing for its greasy skin feel and skin irritation. A further object of the present invention, therefore, is to provide O-W sunscreen emulsion compositions having pristine ZnO particles dispersed in the water phase, which exhibit higher SPF values as compared to conventional O-W sunscreen emulsions, for a given loading of organic UV absorbers contained therein.